Hmmm. My gut tells me to say watergate, on the grounds that it brought about a major change in the american political scene, media, and psyche, whereas Kennedy surviving may not change that much. On the other hand, a Kennedy administration would be somewhat different from a Johnson one, with ripple effects down the line, and his assassination was probably the more traumatic event for the nation.
I vote kennedy, on the grounds that his survival is further removed from the present (more room for butterflies), and that it set the stage for the political environment which eventually created watergate.